Meteor Shower to Peak on Saturday

John Roachfor National Geographic News

December 11, 2002

If clouds and a bright moon obscured your view of last month's Leonidsmeteor shower, take heart. Meteor gazers have another chance to catch ashow this year. The annual Geminid meteor shower is reaching peak force,streaking from a mysterious object known as 3200 Phaethon.

The shower began on December 7 and is predicted to reach peak intensity of 120 meteors per hour at 10:00 Universal Time (5:00 a.m. ET) on December 14, according to Bill Cooke, an astronomer at the Marshall Spaceflight Center in Huntsville, Alabama.

While the Geminids can be seen in the Northern Hemisphere from 10:00 p.m. local time on, astronomers suggest the best time to view the shower is from 2:00 a.m. until dawn. That is the time when the local sky is pointed directly into the meteor stream.

The Geminid meteor shower originates from the constellation Gemini. Astronomers consider it to be one of the best each year, topping even the more well-known Perseids, which occur in August.

In addition, the Geminids hold the allure of being something out of the "Twilight Zone," said Cooke. Astronomers are not sure whether to call the source of the meteor shower, 3200 Phaethon, a comet, an asteroid, or something else yet defined by science.

"It has become obvious in recent years that our nomenclature in astronomy is not keeping pace with the discoveries," said Cooke. "We like to place objects in nice, well-defined categories, even in light of the knowledge that Mother Nature makes no such distinctions."

Defining Geminids

Most meteor showers result from debris that boils off a comet's nucleus when it passes close to the sun. This debris orbits the sun along with the comet, forming a long, thin stream of debris. When this debris hits Earth's atmosphere, it burns up, an event popularly known as a shooting star.

When the Geminids suddenly appeared in the night sky in 1862, astronomers started searching for the comet that was the source of the meteor shower. For more than a century, astronomers found nothing. Then, in 1983, scientists using NASA's Infrared Astronomical Satellite discovered a curious object moving in the same orbit as the Geminid meteors.

There was just one catch: the object didn't look like a comet. Rather, it was several kilometers wide and looked more like a rocky asteroid. Indeed, 3200 Phaethon has an orbit that is characteristic of an asteroid and its meteors are about four times denser than meteors produced by well-known comet showers, said Cooke.

Brian Marsden, an astronomer with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, suggests that 3200 Phaethon could be bombarded by other rocks as it passes through the main asteroid belt, causing a tail of ice and dust.

This phenomenon is thought to be the cause of the comet-like tail associated with Elst Pizarro, an asteroid discovered in 1996, said Marsden. This could also be the cause of 3200 Phaethon's debris tail. "Although it is difficult to see how the rock-like dust could persist to yield Geminids annually, with records from centuries ago," he said. "So that brings us back to the comet idea."

Recent studies on the orbits of the Geminid meteors indicate that they were produced when 3200 Phaethon passed closest to the sun, which is comet-like behavior.

"I actually believe that 3200 Phaethon is a comet, albeit an inactive one. [But] because it doesn't look like a comet, we have classified it as an asteroid," said Marsden.

Cooke holds to his theory that 3200 Phaethon is an object that cannot be defined with current astronomical terms.

"Phaethon may well be a 'Twilight Zone' object, intermediate between asteroids and comets. In which case, both sides of the debate would be part right and part wrong," Cooke said. "This is my current opinion."

Collision Course?

Regardless of whether 3200 Phaethon is indeed an asteroid or a comet, it is classified as a potentially hazardous asteroid. Its orbit brings the body close enough to Earth that scientists believe it has a chance of actually making impact.

To be classified as a potentially hazardous asteroid, a body must pass within 4.6 million miles (7.5 million kilometers) of Earth. There are currently 480 known potentially hazardous asteroids, according to NASA.

Every year in mid-December when the Geminid meteor shower is active, Earth is less than 2 million miles (3.2 million kilometers) from Phaethon's orbit.

The closest upcoming approach for 3200 Phaethon is to 1,812,640 miles (2,917,162 kilometers) on December 14, 2093, said Marsden, making contact in the foreseeable future unlikely. The distant future, however, is less certain.

"I don't know that anyone has specifically made long-term calculations for Phaethon, but over a million years the chance of a hit would be rather good, judging by what we know about similar bodies," he said.